1. Field of the Invention
This invention relates in general to vacuum deposition devices and in particular to a new and useful apparatus for determining the rate of flow of particles in a deposition device during the coating of a substrate.
2. Description of the Prior Art
By rate of particle flow is meant the quantity of particles flowing within one second through a predetermined cross-sectional area of the particle current. In the deposition of thin layers on substrates, the measuring of the rate of particle flow is of particular importance since the properties of the layer (structure, constitution, variation of the material composition of the layer due to chemical reactions of the particles during the deposition, etc.) are appreciably influenced by the velocity of deposition so that, for a reproducible application of thin layers, a continuously uniform rate of the particle flow is necessary. Usually, the particle currents needed for the application of thin layers are produced by thermic evaporation or by atomization of the material to be deposited, effected by ion bombardment (for example, by cathode sputtering in rare gases). The substrates to be coated are positioned in the particle current. The rate of condensation on the substrate depends not only on the rate of particle flow but, in addition, on the coefficient of condensation.
The devices for deposition plants, up to date known as ratemeters, are based on the measurement of the vapor density .rho. in the evaporation current in the space between the vapor source and the substrates. Usually, this density is determined by measuring the vapor pressure by means of an ionization manometer into which the vapor particles are directed. Its relation to the evaporation rate R is given by the formula EQU R = .rho. .multidot.v
where v is the average velocity of the vapor particles in the direction from the source to the substrate. This velocity has not been measured in vacuum-deposition devices as yet. The use of a vapor density measuring instrument as a ratemeter was based on the assumption that, in a certain evaporation process in a given device, a measured vapor density is always associated with a definite velocity v which remains constant from measurement to measurement. This assumption, however, in general, is not correct with a sufficient accuracy so that a secure measuring of the rate merely by measuring the vapor density is not possible.
In addition, in the known measuring instruments, which must be located within the deposition device, disturbances occur frequently insofar as parts of the measuring equipment, for example, the electrodes of an ionization manometer, become coated themselves.
Another kind of known ratemeter uses the variation of the natural frequency of an oscillator quartz located in the device, which frequency variation results from the coating and is determined by means of electronic measuring equipment. From the frequency variation, the mass deposited per time unit on the quartz surface and, therefrom, the number of particles passing to condensation may be determined. These devices have the disadvantage that they make possible the measuring of the rate of flow only during a limited period of time since the layer deposited on the quartz must not exceed a certain thickness.
Further drawbacks of the known oscillator-quartz ratemeters are the disturbances which are caused by the heat radiation of the evaporation source and by the condensation heat which is released on the quartz surface during the coating thereof. In additon, oscillator-quartz-measuring equipment are susceptible to troubles caused by electric gas discharges which occur in deposition devices, primarily in the various processes of the atomization of solid matter by ion bombardment.